I don't know why the buttons are disabled however from what I can tell you should still be able to play WoL online without HoTS - what do the tool-tips for the disabled buttons say?
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JustinMar 25 '13 at 12:23

Maybe becusse the game its still downloading all the stuff.. I will wait the others 3GB
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dynamicMar 25 '13 at 12:25

1

check in your settings if HoTS is selected, in HoTS, I have a popup that says I can switch between versions of the game in the options.
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Hugo TrudelMar 25 '13 at 12:44

@HugoTrudel: you were right, if you want make an answer
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dynamicMar 25 '13 at 14:24

@Justin the disable buttons doesn't say anything but after you log in to your account it says to upgrade to get full version of starcraft to unlock those Macthmaking Versus A.I and stuff
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user48884May 21 '13 at 1:07

Also be aware that Blizzard enabled "spawning" a while ago, which will automatically bring all players up to the highest level of anyone in their party. So if you play in a group with someone who has HoTS, you will have access to the HoTS ladder and units.